1. Field of the Invention
This invention relates to the production of aluminum alloy sheet for the automotive industry, particularly for body panel applications, having excellent bendability, together with good paint bake response and recyclability.
2. Description of the Prior Art
Various types of aluminum alloys have been developed and used in the production of automobiles, particularly as automobile body panels. The use of aluminum alloys for this purpose has the advantage of substantially reducing the weight of the automobiles. However, introduction of aluminum alloy panels creates its own set of needs. To be useful in automobile applications, an aluminum alloy sheet product must possess good forming characteristics in the as-received condition, so that it may be bent or shaped as desired without cracking, tearing or wrinkling. In particular, the panels must be able to withstand severe bending, as occurs during hemming operations, without cracking. Hemming is the common way of attaching outer closure sheets to underlying support panels and results in the edges of the sheet being bent nearly back on itself In addition to this excellent bendability, the aluminum alloy panels, after painting and baking, must have sufficient strength to resist dents and withstand other impacts.
Aluminum alloys of the AA (Aluminum Association) 6000 series are widely used for automotive panel applications. It is well known that a lower T4 yield strength (YS), and reduced amount of Fe, will promote improved formability, particularly hemming performance. A lower yield strength can be achieved by reducing the solute content (Mg, Si, Cu) of the alloy, but this has traditionally resulted in a poor paint bake response, less than 200 MPa T8 (0% strain). This poor paint bake response can be countered by increasing the gauge, or by artificially aging the formed panels. However, both of these approaches increase the cost and are unattractive options. Furthermore, a reduced Fe content is not sustainable with the use of significant amounts of scrap in the form of recycled metal. This is because the scrap stream from stamping plants tends to be contaminated with some steel scrap that causes a rise in the Fe level.
Furthermore, the necessary material characteristics of outer and inner panels are sufficiently different that the natural trend is to specialize the alloys and process routes. For example, an AA5000 alloy may be used for inner panels and an AA6000 alloy for outer panels. However, to promote efficient recycling it is highly desirable to have the alloys used to construct both the inner and outer panel of a hood, deck lid, etc. to have a common or highly compatible chemistry. At the very least, the scrap stream must be capable of making one of the alloys, in this case the alloy for the inner panel.
In Uchida et al. U.S. Pat. No. 5,266,130 a process is described for manufacturing aluminum alloy panels for the automotive industry. Their alloy include as essential components quite broad ranges of Si and Mg and may also include Mn, Fe, Cu, Ti, etc. The examples of the patent show a pre-aging treatment that incorporates a cooling rate of 4° C./min from 150° C. to 50° C.
In Jin et al. U.S. Pat. No. 5,616,189 a further process is described for producing aluminum sheet for the automotive industry. Again, alloys used contain Cu, Mg, Mn and Fe. The aluminum sheet produced from these alloys was subjected to a 5 hour pre-age treatment at 85° C. The disclosure furthermore states that the sheet can be coiled at 85° C. and allowed to cool slowly to ambient at a rate of less than 10° C./hr. The aluminum sheet used in this patent was a continuous cast (CC) sheet and sheet products produced by this route have been found to exhibit poor bendability.
It is an object of the present invention to provide an improved processing technique whereby an aluminum alloy sheet is formed which has excellent bendability.
It is a further object of the invention to provide an aluminum alloy sheet product having good paint bake response.
It is a still further object of the invention to provide an aluminum alloy sheet product which is capable of being recycled for use in the production of automotive body panels.